Downsizing and weight-saving of electronic equipment requires higher magnetic properties of the magnets used therein. In particular, active development has been made of R2Fe14B type rare earth sintered magnets, which have a high magnetic flux density. Such R2Fe14B type rare earth sintered magnets are generally produced by melting and casting the starting materials, pulverizing the resulting magnet material alloy, compacting the pulverized alloy in a magnetic field, followed by sintering and ageing.
The raw material alloy for R2Fe14B type rare earth sintered magnets usually contains dendrites of a R2Fe14B phase (sometimes referred to as a 2-14-1 phase hereinbelow) and a region of a phase with a relatively low melting point having a higher rare earth metal content than the 2-14-1 phase (sometimes referred to as an R-rich region hereinbelow). In the production of R2Fe14B type rare earth sintered magnets, when the raw material alloy is sintered, the R-rich region melts into a liquid phase and fills gaps between the grains of the 2-14-1 phase, to thereby improve the sinterability and contribute to densification of the resulting sintered product. When solidified, the non-magnetic R-rich region coats the grains of the ferromagnetic 2-14-1 phase to magnetically insulate the 2-14-1 phase and improve the coercivity.
For producing a raw material alloy for such sintered magnets, there is conventionally known a method for casting an alloy having a structure with finely dispersed R-rich region, by rapid solidification, such as strip casting (Patent Publication 1).
Patent Publication 1 discloses that the raw material alloy wherein the R-rich region is finely dispersed, has good pulverizability, and as a result, the grains of the 2-14-1 phase are uniformly coated with the R-rich region after sintering, which improves the magnetic properties.
Patent Publication 2 teaches that microscopic analysis of the crystal structure of alloy flakes indicates that fine dendritic or columnar crystals in the flakes have an impact on oxidation in finely pulverizing the flakes into magnet powders and on decrease in the degree of orientation of the resulting sintered magnet. In order to reduce the fine dendritic or columnar crystals, this publication proposes a method of producing a raw magnet alloy including controlling the temperature of the alloy melt in rapid solidification, the primary cooling rate on a cooling roll, and the secondary cooling rate after the flakes are separated from the cooling roll.
Patent Publications 3 and 4 disclose that the magnetic remanence is improved by increasing the volume ratio of the 2-14-1 phase in the raw material alloy, reducing the intervals between the R-rich regions, and increasing the crystal grain size in the alloy structure, and that, as an example, a preferred alloy has an average crystal grain size of 10 to 100 μm, and intervals between the R-rich regions of 3 to 15 μm. The publications also disclose that such a raw material alloy may be produced by controlling the primary cooling rate in the rapid solidification, the secondary cooling rate, and the temperature of heat treatment.
By increasing the primary cooling rate, the intervals between the R-rich regions are reduced, and the crystal grain size is also reduced, whereas by reducing the primary cooling rate, the intervals between the R-rich regions are increased, and the crystal grain size is also increased. On the other hand, by controlling the secondary cooling rate, i.e. by decreasing the cooling rate after solidification, the intervals between the R-rich regions may be increased under certain conditions.
However, mere control of the primary and secondary cooling rates and the heat treatment cannot achieve both increase in ratio of the dendrites of the 2-14-1 phase and prevention of the chill crystal formation at the same time. Further, increase in the crystal grain size is limited and the desired crystal grain size cannot be achieved.
With regard to the cooling roll, Patent Publication 5 discloses a cooling roll having gas channels formed thereon as ventilating means. The width of the gas channels disclosed in Examples is 20 μm or less, and the alloy intended to be produced with the roll has an amorphous or microcrystalline structure. Patent Publication 5 does not even imply that, using such a cooling roll, alloy flakes may be produced which have the alloy structure containing the R-rich region and the dendrites of the 2-14-1 phase with the dendrite content of not lower than 80 vol %.
Patent Publication 6 discloses a cooling roll having grooves circumferentially extending in the Cr surface layer, wherein the average distance between the grooves is 100 to 300 μm in an arbitrary sectional surface containing the shaft, the peaks and troughs in a cross section of the grooves are smoothly connected, the mean center line roughness is 0.07 to 5 μm, and the depth of the grooves is 1 to 50 μm. The peaks and troughs of the grooves are smoothly connected so as to configure the grooves to expand the surface area of the cooling roll, and allow entry of the alloy melt also into the troughs of the grooves to improve the contact between the alloy melt and the roll. Thus using this cooling roll, alloy flakes cannot be obtained having the alloy structure wherein the chill crystal content is suppressed and the intervals and size of dendrites are uniform.
Patent Publication 7 discloses to produce alloy flakes for a rare earth sintered magnet, using a cooling roll which has a plurality of linear convexes and concaves crossing with each other in the surface of the roll, and has a ten-point mean roughness of not lower than 3 μm and not higher than 30 μm. Such convexes and concaves may inhibit random formation of an area wherein the R-rich region is extremely fine (fine R-rich region) on the side of the cooling roll. However, using this cooling roll, alloy flakes cannot be obtained which have the alloy structure containing the R-rich region and the dendrites of the 2-14-1 phase with the dendrite content of not lower than 80 vol %, and have uniform intervals and size of the dendrites.    Patent Publication 1: JP-5-222488-A    Patent Publication 2: JP-8-269643-A    Patent Publication 3: JP-9-170055-A    Patent Publication 4: JP-10-36949-A    Patent Publication 5: JP-2002-50507-A    Patent Publication 6: JP-5-269549-A    Patent Publication 7: JP-2004-43921-A